As semiconductor integrated circuits become more microscopic and more integrated, insulating films, metal films and other films formed on a substrate (such as a semiconductor substrate) are desired to have less thickness, better coverage for complex shapes, macroscopic uniformity over the whole wafer and microscopic smoothness at the nanometer level. However, the conventional CVD process cannot satisfy at least a portion of the above desires.
At present attention is paid to the ALD (Atomic Layer Deposition) process as a film forming process that can satisfy the above desires. In the ALD process, each of different source gasses is supplied one by one, supplied source gasses are absorbed by a reactive surface and form a film at an atomic layer or molecular layer level, and get a thin film having a desired thickness by repeating the above process.
In more detail, a first source gas is supplied to the substrate to form its absorption layer on the substrate. Then, a second source gas is supplied to the substrate to cause them to react. In this process, the first source gas reacts with the second source gas after being absorbed, and therefore the temperature of the formed film can be lowered.
When forming a film at a hole, the ALD process can avoid coverage degradation problems resulting from the source gas reaction above the hole, which problem cannot be solved by the conventional CVD process.
The thickness of the absorption layer is generally a single layer, or at most two or three layers, of atoms and molecules, and is determined by its temperature and pressure. The absorption layer has self-controllability in which more than the necessary source gas for forming the absorption layer is exhausted out, and therefore the ALD process is suitable for controlling a very thin film. And each time the film forming process is performed at the atomic or molecular level, the reaction is easy to progress, and almost no impurity is left in the film, and therefore the ALD process is preferable.
However, the ALD process has a problem in that the total time required for forming a film becomes long because each time the film forming process is performed at the atomic or molecular level.
The ALD process has an incubation period in the first several cycles, during which no film is formed, that has seriously lowered throughput.